| Abstract
| - The thermodynamics of Fe3+ to Fe2+ reduction for the five-coordinate high-spin native form of horseradish peroxidase and for its six-coordinate low-spin cyanide adduct have been determined from variable-temperature UV−vis spectroelectrochemical experiments. In both cases, the ΔH°‘rc and ΔS°‘rc values are positive. Hence, the negative reduction potentials turn out to be the result of two opposing and partially compensating contributions: a large enthalpic term, which is the determinant of the negative E°‘ values for both species, and a smaller, yet relevant, entropic contribution. The decrease in E°‘ of the Fe3+/Fe2+ couple on cyanide binding turns out to be a fully entropic effect, unequivocally demonstrating the importance of entropic effects in determining the E°‘ values of redox metal centers.
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